Ive looking into a lot of tuners such as the diablosport predator. I know there is a lot of info on here about them, but here is my question: if these things can give you more power and better gas mileage with no side affects, why don't the car companies just do it prior to selling the vehicle?

The car companies try to set the tuning as safe as possibe. They over design to the safeness in the tune to limit possible harm over the life of the car. This typically leaves a large area for improvement from tuning, whether a handheld unit or software.

In today's marketplace with advanced electronics and sensors it is a lot harder to make power compared to 5 or 10 years ago. Just a tune on a 2010 Camaro does not pickup power like a Gen4.

You have to remember that a vehicle designed for production requires safeguards, and for parts to last 100,000 miles without premature failure due to warranty claims. This does not mean that performance and tuning can harm your car but it is a lot easier to dial in a car more specific for performance when calibrations and parts are not being designed for 100,000 different types of drivers and consumers

Auto makers also have to meet emissions guidelines. Not saying tuning your camaro will make it fail an emissions test, but rather the strict rules set it place for them to follow. So with that being said there is some room left on the table for better efficiency that a skilled calibrator should be able to find.

Factory calibrations are tested for several conditions as the vehicle ages to maintain these guidelines.